Intraluminal prostheses used to maintain, open, or dilate blood vessels are commonly known as stents. Stent constructions generally include lattice type cylindrical frames that define a plurality of openings. Common frameworks for stents include, for example, individual rings linked along the length of the stent by a linking member, a continuous helically wrapped member (that may include one or more linking members), a braid or a mesh formed into a tubular structure, and a series of interconnected struts. Stents may be formed by arranging one or more members in a pattern along a longitudinal axis to define essentially a cylinder and connecting the one or more members or otherwise affixing them in position (e.g., interconnecting with a filament). Stents may also be formed by cutting openings into a tube of material (e.g., shape memory).
Stents may have self-expanding and/or balloon expandable properties. Self-expanding stents are delivered to a blood vessel in a collapsed condition and expand in vivo following the removal of a constraining force and/or in the presence of an elevated temperature (due to material properties thereof), whereas balloon expandable stents are generally crimped onto a balloon catheter for delivery and require the outwardly directed force of a balloon for expansion. Stents can be made of various metals and polymers and can include a combination of self-expanding and balloon expandable properties.
Synthetic vascular grafts are routinely used to restore the blood flow in patients suffering from vascular diseases. For example, prosthetic grafts made from expanded polytetrafluoroethylene (ePTFE) are commonly used and have shown favorable patency rates, meaning that depending on a given time period, the graft maintains an open lumen for the flow of blood therethrough. Grafts formed of ePTFE include a microstructure characterized by spaced apart nodes connected by fibrils, the distance between the nodes defined as internodal distance (IND), and are generally extruded either as a tube or as a sheet or film that is fashioned into a tube. Grafts can also be created from fibers woven or knitted into a generally tubular shape.
It is known in the art to use stents in combination with vascular grafts to form stent-grafts. Because stent-grafts are often intraluminally deployed in vessels of varying sizes and tortuosity, flexibility can be an important consideration. Flexibility can be imparted to a stent-graft in a variety of ways, including, for example, connection of the stent to the one or more graft layers, configuration of the stent and/or graft layer(s), spacing of the stent struts, rings, or members along the length of the graft(s), etc. For example, U.S. Pat. No. 6,398,803 and U.S. Pat. No. 6,770,087 to Layne et al. describe a graft layer with openings to enhance flexibility. Another important consideration in the design of a stent-graft is the ability of the stent to withstand stress and fatigue, caused, for example, by plastic deformations occurring at strut junctions when the stent is subjected to circumferential forces. Stent strength can be enhanced through material choice, stent configuration, arrangement and configuration of graft layers, connecting members between stent members, etc. Another consideration in the design of certain stent-grafts is properties to resist kinking of the stent-graft. For example, when a stent-graft is positioned in a bend in a blood vessel or bypass graft, depending on the acuteness of the angle of the bend, the stent-graft can potentially kink and thereby become unsuitable for passage of blood therethrough.
One example of an allegedly flexible and kink resistant stent-graft is described in U.S. Pat. No. 6,042,605 to Martin et al., the stent-graft formed by helically arranging an undulating stent member about a graft member, interweaving a linking member between undulations in adjacent turns of the helical member, and helically arranging a coupling member in the form of a flat ribbon or tape around the assembly. Another example is provided in U.S. Pat. No. 6,312,458 to Golds, the stent-graft formed from an elongate wire helically wound about a graft member at a first angle and an elongate securement member helically wound over both the stent and graft members at a second angle not congruent to the first angle. Such a stent-graft is alleged to be an improvement over the Martin et al. stent-graft both because the use of a broad coupling member is said to decrease the overall flexibility of the stent-graft and because wrapping the coupling member at the same angular orientation of the stent is said to decrease flexibility and expandability of the stent. In each of these examples, however, the outermost layer is a thin tape, ribbon, thread or suture, rather than a support member, such that the radial strength of the stent-graft is limited.
The following references relate to stent-grafts: U.S. Pat. No. 5,667,523 to Bynon et al.; U.S. Pat. No. 6,042,605 to Martin et al.; U.S. Pat. No. 6,264,684 to Banas et al.; U.S. Pat. No. 6,312,458 to Golds; U.S. Pat. No. 6,361,637 to Martin et al., U.S. Pat. No. 6,398,803 to Layne et al.; U.S. Pat. No. 6,520,986 to Martin et al., U.S. Pat. No. 6,652,570 to Smith et al.; U.S. Pat. No. 6,673,103 to Golds et al.; U.S. Pat. No. 6,770,087 to Layne et al.; U.S. Pat. No. 6,881,221 to Golds; U.S. Pat. No. 6,911,040 to Johnson et al.; and U.S. Pat. No. 6,945,991 to Brodeur et al., each of which is incorporated by reference in its entirety into this application.
Applicants have recognized that it would be desirable to provide a stent-graft that is able to combine flexibility, kink-resistance and good radial strength, embodiments of which are described herein along with methods of making same.